1. Field of the Invention
The present invention relates generally to high power semiconductor diode laser configurations, and more particularly to a diode laser oscillator integrated monolithically with an optical power amplifier by way of a controlling or pre-amplifying section.
2. Description of the Prior Art
Semiconductor diode lasers are useful for a wide variety of applications, such as fiberoptic communications and optical data retrieval. Some potential applications, such as optical space communications, optical recording, semiconductor processing, spectrum analysis, laser radar, range finding and contour mapping require high power, single spectral and spatial mode optical sources. Unfortunately, presently available diode laser configurations have difficulty in achieving both high power and a diffraction limited single mode beam in a single device. Diffraction limited single mode lasers are commercially available with an output power of 100 mW cw.
Phased array lasers are capable of achieving higher power. For example, a 10 W cw laser bar is commercially available. However, phased array lasers in general, tend to have poor modal discrimination due to a variety of causes, including gain non-linearities and spatial hole burning. These lasers operate in a diffraction limited single mode at low to moderate output powers, but as multimode oscillators at high power, some even at currents only slightly above threshold. Laser arrays that are most successful at maintaining a diffraction limited output do so only by suppressing other more favored modes, thereby limiting the achievable output.
Monolithic master oscillator power amplifiers (MOPAs) capable of delivering respective power outputs of 1 W without the poor modal discrimination of the phased array laser are disclosed by the following patents:
U.S. Pat. No. 4,744,089. Patentee: Montroll et al. Issued: May 10, 1988. PA1 U.S. Pat. No. 5,003,550. Patentee: Welch et al. Issued: Mar. 26, 1991. PA1 U.S. Pat. No. 4,747,650. Patentee: Sakuda. Issued: May 31, 1988. PA1 U.S. Pat. No. 4,856,017. Patentee: Ungar. Issued: Aug. 8, 1989. PA1 U.S. Pat. No. 4,977,567. Patentee: Hanke. Issued: Dec. 11, 1990. PA1 U.S. Pat. No. 5,019,787. Patentee: Carlson et al. Issued: May 28, 1991. PA1 Bendelli et al. "A New Structure for High-Power TW-SLA". IEEE Photonics Technology Letters, Vol. 3, No. 1. Published: January, 1991. pp. 42-44. PA1 Koren et al. "Wavelength Division Multiplexing Light Source with Integrated Quantum Well Tunable Lasers and Optical Amplifiers". AT&T Bell Laboratories. Accepted for Publication: Mar. 13, 1989. PA1 Ikeda et al. "Transverse Mode Controlled Wide-Single-Stripe Lasers by Loading Modal. Filters". SPIE Vol. 1043 Laser Diode Technology and Applications (1989). pp. 81-86.
U.S. Pat. No. 4,744,089 discloses a MOPA with a laser diode or oscillator, the laser diode having a periodic grating providing distributed feedback. A power amplifier with a diverging active area follows the laser, and the amplifier output facet is made anti-reflecting. The laser oscillator and amplifier are formed with separate electrical contacts so they can be driven by independent current sources.
U.S. Pat. No. 5,033,550 discloses a MOPA comprising a single mode diode laser with distributed Bragg reflectors, an amplifier in tandem with the laser, lateral phase controllers and a detuned second order grating surface output coupler, all on a common substrate. The lateral phase controllers are an array of separately addressable electrodes that adjust the optical path length to compensate for phase variation in the amplifiers and also to provide lateral steering of the output beam.
While the MOPA disclosed by Montroll et al. is capable of amplifying the laser beam to a desirable power level, it does not disclose suitable means for controlling the laser beam. More particularly, U.S. Pat. No. 4,744,089 only contemplates one way to control the laser beam, i.e. selectively modulating the laser beam at it source by selectively pumping current in the master oscillator. This sort of selective current pumping, however, can lead to "chirpping" in which the wavelength of the laser is shifted undesirably during the modulation process. Even though the MOPA disclosed by Welch et al. is well suited to adjust for inhomogeneities and non-linearities in the amplifier section, it fails to disclose any means for controlling the laser beam exiting the oscillator. Consequently, the degree to which the a laser beam can be controlled by the Welch et al. MOPA is limited severely. It would be desirable to provide a simply designed MOPA, such as the MOPA disclosed by Montroll et al, having a section adapted to effect a broad range of suitable control operations on the laser beam.
References, which may be pertinent to the consideration of various aspects of the present invention, are listed below:
A brief summary of the above references follows below:
U.S. Pat. No. 4,747,650 discloses a semiconductor optical amplifier comprising three semiconductor waveguides, each of which is fabricated on the same semiconductor chip. The first waveguide transmits an optical signal, while the third waveguide forms an optical coupling with the first waveguide and operates as an optical amplifier.
U.S. Pat. No. 4,856,017 discloses a semiconductor laser having an elongated active gain medium layer with a wide output facet at one end and a relatively narrower facet at the other end. Means are provided between the facets for providing frequency dependent feedback for the gain layer. The layer is pumped for stimulating emission of radiation, with the pumped area having parallel edges adjacent to the smaller facet and diverging toward the wider output facet.
U.S. Pat. No. 4,977,567 discloses a semiconductor laser arrangement for high output power in a lateral fundamental mode. The arrangement comprises a laser region, including a waveguide, as well as a coupling region, and an intensification region, the laser, coupling and intensification regions being integrated on a common substrate. Contacts are respectively coupled with the substrate, the laser region and the intensification region for pumping current to the arrangement.
U.S. Pat. No. 5,019,787 discloses an optical amplifier comprising a body of semiconductor material having means for receiving a beam of light and directing the beam along a path in the body. A gain section is provided along the beam path though the body, the gain section being capable of amplifying the beam. The body also includes means for receiving the amplified beam from the gain section and directing the amplified beam out of the body.
The article entitled "A New Structure for High-Power TW-SLA" discloses a structure, comprised of an exponentially tapered active layer, for traveling wave semiconductor laser amplifiers (TW-SLA) to improve the saturation output power and the quantum efficiency thereof.
The article entitled "Wavelength Division Multiplexing Light Source with Integrated Quantum Well Tunable Lasers and Optical Amplifiers" discloses a photonic integrated circuit composed of three 1.5 .mu.m wavelength multiple quantum well tunable lasers with a passive optical output power combiner and an optical output amplifier.
The article entitled "Transverse Mode Controlled Wide-Single-Stripe Lasers by Loading Modal Filters" discloses two kinds of modal filters for high power diode lasers with a wide stripe.